Low voltage field emission scanning electron microscope

ABSTRACT

A Field Emission Scanning Electron Microscope having a field emission tip for generating charged particles and being axially aligned with an electrode and with an intermediate electrode interposed there between. The electrode and the intermediate electrode forming a field lens for focusing the charged particles from the field emission tip. Appropriate voltages are applied for providing extraction, acceleration and focus fields. The electrode potential being lower than the intermediate electrode potential with respect to the charged particles and selected to provide a real focus of the charged particles at a position outside the lens field and distal of the field emission tip thereby producing an intense beam of low energy particles.

United States Patent 91 Coates et a1.

[ Jan. 8, 1974 LOW VOLTAGE FIELD EMISSION SCANNING ELECTRON MICROSCOPE American Optical Corporation, Southbridge, Mass.

Filed: Aug. 16, 1971 Appl. No.: 171,815

Assignee:

US. Cl. 250/49.5 C, 250/49.5 A Int. Cl. H01] 37/26 Field of Search 250/495 A, 49.5 C

References Cited UNITED STATES PATENTS 10/1940 Sukumlyn 250/495 C 7/1942 Ramo 250/495 C 5/1963 Benway 250/495 C l/l952 Grivet et al. 250/495 C 3/1953 Bruck 250/495 C OTHER PUBLICATIONS A Simple Scanning Electron Microscope" by Crewe .7 Ctbieiqnm et al. The Review of Scientific Instruments Vol. 40, No. 2 Feb. 1969, pp. 24l246.

Primary Examiner-James W. Lawrence Assistant Examiner-Harold A. Dixon Attorney-Howard R. Berkenstock, Jr. and William 5 7 ABSTRACT A Field Emission Scanning Electron Microscope having a field emission tip for generating charged particles and being axially aligned with an electrode and with an intermediate electrode interposed there between. The electrode and the intermediate electrode forming a field lens for focusing the charged particles from the field emission tip. Appropriate voltages are applied for providing extraction, acceleration and focus fields. The electrode potential being lower than the intermediate electrode potential with respect to the charged particles and selected to provide a real focus of the charged particles at a position outside the lens field and distal of the field emission tip thereby producing an intense beam of low energy particles.

16 Claims, 4 Drawing Figures I' |i r "gt- 1 I 26 27 20 2| I I I I 19 J23 l'\ m 2e I I l I l l video receiver PATENTEDJAN 8 1974 sum 10? 2 video receiver FIG. I

FIG]:

INVENTORS Vmcent J. Coates Leonard. M. Welter BY PATENTED JAN 8 I974 33. 784.8 1 5 sum 2 BF 2 INVENTORS Vmcenf J. Comes Leonard M. Welter Afloey 1 LOW VOLTAGE FIELD EMISSION SCANNING ELECTRON MICROSCOPE BACKGROUND OF THE INVENTION The invention relates in general to electron optical systems and more particularly to a field emission Electron Gun capable of producing a low voltage intense beam of focused electrons. In the applicants cpending patent application Ser. No. 46,425 filed June 15, 1970 now U.S. Pat. No. 3,678,333 there is described a Scanning Electron Microscope employing a field emission tip as its source of electrons. The use of the field emission tip allows the formation of a high intensity focused beam of charged particles providing the intense illumination needed in scanning microscopy. This co-pending application is principally directed toward a system which will provide and maintain the high vacuum requirements (in the neighborhood of to the '-9 torr) and high voltage protection necessary to acheiving a reliable high resolution Field Emission Scanning Electron Microscope. Normally the accelerating voltages used in the operation of the described Scanning Electron Microscope as well as other similar devices presently used in the art ranges between 1,000 and 100,000 volts. The same range of accelerating voltages is applied and used in conventional Scanning Electronic Microscopes using thermionic or filament guns for supplying the necessary electrons. As may be readily appreciated, the use of such high accelerating volt ages especially in the neighborhood of 100,000 volts produces a myriad of problems. The specimen is often damaged by beams containing such high energy particles and in many instances the morphology of the specimen is so seriously altered as to create doubt as to the validity of the information obtained. In yet other instances when studying nonconductive surfaces the use of high energy beams will cause charge up" conditions i.e., electrostatic charge build up on the surface from the electrons not being conducted away. In the instance of conventional thermionic guns, attempts to lower the accelerating voltage produces a drastic reduction in beam current intensity due to electron optical effects and the spot size increases, both ultimately producing a loss of desired resolution. In the case of conventional Scanning Electron Microscopes of the Field Emission type while markedly improved from an intensity and spot size point of view, the focus of the beam becomes a virtual image or a real image withinthe field of lenses and thus cannot impinge upon the specimen to be investigated. The applicants have discovered a novel means of producing a real image of an intense beam of low energy particles having a focus plane in the specimen holding portion of the Scanning Electron Micro scope.

It is therefore an object of this invention to produce a low voltage Field Emission Scanning Electron Microscope producing a low energy high intensity beam of charged particles. It is another object of this invention to provide a method for producing a low energy high intensity beam of charged particles in a Scanning Electron Microscope. It is another object of this invention to provide a Field Emission Scanning Electron Microscope capable of producing a low energy high intensity beam of electrons. It is another object of this invention to provide a Field Emission Scanning Electron Microscope having a greatly reduced propensity for high voltage breakdown. It is yet another object of this invention to provide a Scanning Electron Microscope for viewing specimens susceptible to beam damage. Yet, another object of this invention is to provide a Scanning Electron Microscope particularly useful in mirror scanning of a specimen.

SUMMARY OF THE INVENTION As contemplated by the applicants invention, there is provided a field emission tip gun for supplying or generating charged particles to be normally employed in a Scanning Electron Microscope. A final electrode is axially aligned with the field emission tip and another intermediate electrode is interposed between the field emission tip and the'final electrode. The final electrode and the intermediate electrode together form a field lens which acts upon the beam produced by the charged particles generated from the tip. Essentially, the field lens substantially focuses the charged particles to a spot in a plane useful for viewing a specimen under investigation. A source of electrical potential supplies voltages to the field emission tip, the final electrode and the intermediate electrode for providing extraction, acceleration and focus fields necessary to formation of a charged particle beam and its ultimate focus.

The electrode potential is lower than the potential of the intermediate electrode with respect to the charged particles forming the beam and is selected to provide a real focus of the charged particles at a position outside of the lens without a need for further lensing. This beam focus is at a point distal of the field emission tip and provides an intense beam of low energy particles for viewing the specimen.

In another aspect of the applicants invention there is contemplated a method for producing a low energy high intensity beam of charged particles from a Field Emission Gun employing a field emission tip as the source of the charged particles. In conformance with this method, extraction accelerating and focusing potentials are applied to an axially aligned field emission tip, final electrode and intermediate electrode. The intermediate electrode being interposed between the field emission tip and the final electrode. The application of these fields serving to form and focus a beam of charged particles generated from the field emission tip. The final electrode potential is selected so as to form a negative lens field with the intermediate electrode with respect to the charged particles thereby producing a real focus of said charged particles outside the lens field and distal of the field emission tip without a need for further focusing. The particles at the spot focus of the beam are of a low energy high intensity character.

For a better understanding of the present invention together with other and further objects'thereof, reference is had to the following description taken in connection with the accompanying 'drawings while its scope will be pointed out in the appended claims. It is emphasized that the preferred embodiments herein described are intended to be illustrative of the applicants invention and in no way delimiting of its scope.

BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 is a functional schematic diagram of a Field Emission Scanning Microscope embodying the applicants invention.

FIG. 2 is a cross sectional view of a typical Field Emission Scanning Electron Microscope.

FIG. 3 is a fragmentary view of an electrode arrangement useful in the practice of the applicants invention.

FIG. 4 is a functional schematic diagram of a specimen being viewed in a particular mode of use of the applicants Field Emission Scanning Microscope.

Conventional symbols are used throughout the drawings and like numbers are used for like or similar parts in each view. These drawings are intended to be illustrative and exemplary of the invention and thus may not always conform to the actual practical embodiments but rather are shown to best point out the applicants novel contribution to the state of the art.

DESCRIPTION OF THE PREFERRED EMBODIMENTS The electron optical system of FIG. 1 is in a form of a Scanning Electron Microscope l embodying therein a field emission gun capable of conforming to the applicants invention. It is significant at this juncture to point out while the Field Emission Gun of the illustrative embodiment is used to produce electrons as the illuminating particles, it is both possible, and in some instances necessary that the same source be used to produce other charged particles such as positive ions for viewing specimens of a particular nature or for obtaining information not readily ascertained from the use of an electron beam. When using a Field Emission Gun as a source of positive charged particles, it is necessary of course that the extraction voltages and the focusing fields be reversed to provide the necessary extraction, acceleration and focusing forces. Ancillary to the Scanning Electron Microscope there is shown a potential source 11 which means provides the various levels of voltages to the electrodes of the Scanning Electron Microscope configuration necessary to the operation of the instrument. A second ancillary unit the video receiver 29, in combination with the scanning and detector portions of the Scanning Electron Microscope 10 displays the desired view of the specimen undergoing investigation.

The field emission tip 21 constitutes the heart of Field Emission Scanning Electron Microscopy systems. It produces the highly coherent high intensity supply of electrons capable of being focused to a spot of desired resolution at the image plane to illuminate the specimen 18. The specimen 18 is shown mounted to a specimen holder 17 which positions the specimen 18 with respect to the focused beam of electrons 13. The beam of electrons or charged particles 13 is primarily formed by the action of anodes 23 constituting an intermediate electrode interposed between the field emission tip 21 and an electrode 24 located distal of intermediate electrode 23 with respect to the field emission tip 21. The electrode 24 and the intermediate electrode 23 form in combination a field l2 acting as a focusing lens on the beam 13 as it passes through the electrode apertures toward the target or specimen 18.

The main accelerating voltage V0 is provided by voltage or potential means 28 to the electrode 24 and its magnitude is selected relative to the potential of field emission tip 21. When used as a Scanning Electron Microscope, the potential of the electrode 24 is normally positive, thereby providing an accelerating voltage to the charged particles; namely, the electrons. The potential source 27 provides a voltage V1 to the intermediate electrode 23 which potential is normally less or lower than the potential of electrode 24 with respect to the charged particles forming the beam 13. Thus, there is provided in normal operation a positive field between the electrode 24 and the intermediate electrode 23 tending to converge the beam 13 toward a focus at the specimen plane 18. A last electrode, namely extraction electrode 22 is placed closest to the field emission tip 21 and like electrode 23 and intermediate electrode 24 has a centrally located aperture axially aligned with the field emission tip 21. The beam 13 is formed by the passage of the charged particles from the field emission tip 21 through the apertures in extraction electrode 22 and intermediate electrode 23 and electrode 24 respectively. The potential source 26 provides a voltage substantially equal to the voltage VI of potential source 27 and provides the extraction field for the field emission tip 21. When used in the electron mode, this voltage is of course positive and produces an extremely high field intensity in the neighborhood of the field emission tip 21 which has an ultimate tip having a radius approximately 1,000A. Of course, it is recognized that when used as a source of positive charged particles then the voltage V must be reversed in potential causing an oppositely polarized field having the necessary intensity to form ions or other charged particles.

The lens system 16 is primarily intended for control of the beam 13 for scanning the specimen 18 in the programmed manner. Essentially, the lens system 16 constitutes deflection plates synchronized to the scanning system of the video receiver 29, thereby locking in the scanning of the specimen l8 and the scanning of the video field of the receiver 29. Detectors 14 and 15 are placed so as to receive or detect information obtained from the surface or through the specimen 18. Information about the specimen 20 may be obtained by detecting transmitted electrons as in the case of a transmission electron microscope, secondary electrons, reflected electrons, absorbed electrons, photons or xrays, any and all of which are generated by the incident electron or other charged particle beam. These detectors 14 and 15 which are most frequently of the scintillation type are used to detect one of the information signals which is then used to modulate the intensity of the synchronously scanned display tube of the video receiver 29 which includes a sweep generator to form an image of the specimen l8. Throughout the remainder of this specification and particularly when considering operative examples or modes of operation, the apparatus will only be considered when used in the electron mode i.e., only when field emission tip 21 is used to generate electron particles. This is not, however, intended to delimit the inherent symmetry of the systems when used in either of the two modes.

The remaining elements of FIG. 1 are a discharge shield 20 and a sublimation coil- 19. The discharge shield 20 finds primary utilization in preventing high voltage discharges from the outer chamber 40 to the extraction electrode 22 and the field emission tip 21, additionally this shield serves as a condensing structure for titanium or other material of suitable character sublimated from the sublimation coil 19. When the sublimated material condenses on the surface of the shield 20, it provides a material which reactively combines with gaseous components in the neighborhood of the field emission tip thereby lowering the vacuum. The sublimator coil 19 is of course part of a vacuum pumping system which in combination with the inherent ion pumping in the neighborhood field emission tip 21 maintains a vacuum in the order of 10 to the 9 to 10 to the l0 torr thereby assuring prolonged and reliable operation of the field emission tip 21 as a source of electrons. It is note worthy to point out that the invention or novel contribution of the applicants contained in this application to some degree obviates the need for the high voltage shield by reducing the accelerating voltage required to form a focused beam of high intensity electrons on the specimen 18 to be viewed.

Before passing to an operational description pointing out the most significant elements of the applicants invention, the Field Emission Scanning Electron Microscope of FIG. 2 will be considered. Disposed within the vacuum chamber 41 and in specifically the high vacuum area 41A, there is the field emission tip 21 comprising the source of electrons. This field emission tip 21 is normally an etched tip which does not employ any filament voltage or power to emit electrons but which rather depends upon the formation of an extraction field in the neighborhood of its ultimate tip which is in the neighborhood of 1,000A radius. When the field developed in this area is sufficiently large so as to overcome the work function of the metal which is normally tungsten and the retarding field developed by emitted electrons, the field emission tip 21 acts as a source for a highly coherent high intensity beam of electrons.

The field emission tip 21 is supported for movement along the X, Y and Z planes and for its removal for replacement. As shown in FIG. 2, the tip support assembly comprises a V-shaped mount 51 which has the tip 21 attached thereto. An insulator disc 54 receives electrode 53 which is attached to the tip 21 which electrode 53 is secured to the insulator disc 54 by a suitable fastener such as threaded member 57. The tip 21, the disc 54 and the electrode 53 are in the form of a plug in assembly which is detachably secured to the distal end of a post 55. The upper end of the insulator post 55 is fixedly secured to a metal shaft having an end cap 60a with a depending flange fixed within the upper end of the post 55. A bellows 61 is welded to the cap 60a to form a vacuum wall or seal with the high vacuum chamber while permitting axial and transverse movement of the tip support assembly 50. The bellows 61 is welded directly to an end cap 62 to complete the vacuum seal. Surrounding the shaft 60 is a sleeve 63 which is located in an opening 64 of the cap 62. Spaced from the cap 62 is a support, plate 65 that has a suitable opening therein for receiving the shaft 60 and the sleeve 63. A knob 68 in threaded engagement with the upper end of the shaft 60 produces axial movement of the tip assembly 50 while adjustment screws 66 tilt the plate 65 thereby producing motion in the X, Y plane for alignment with the apertures of the various anodes of the microscope; namely, extraction electrode 22, the intermediate electrode 23 and electrode 24.

Spaced downstream from the tip 21 in a low vacuum area 41b of the vacuum chamber 41 is the intermediate electrode 23 having a central aperture opening 84 aligned with the field emission tip 21. A second electrode 24 is located further downstream of intermediate electrode 23 and in similar fashion has its aperture 106 axially aliged with the field emission tip 21.

A third electrode or more appropriate discharge shield is diposed within the high vacuum area 41A surrounding or protecting the field emission tip 21 from premature failure due to electrical high voltage discharge. The discharge shield 20 is connected to an output terminal 77 of the high voltage insulator coupling 78. Thus, it is substantially at the same electrical potential as the field emission tip 21 as shown in F IG. 1. The fourth electrode or extraction electrode 22 located within the chamber 41 is disposed substantially within the discharge shield 20. When voltage is applied to the extraction electrode 22, an extraction field is created and electrons are drawn from the field emission tip 21. It is the extraction electrode 22 that permits normal electron emission when the discharge shield 20 is deployed about the field emission tip 21 for protection from excessive voltage transients. The voltages for supplying the proper potentials to the electrodes of the electron gun are provided by a voltage supply 11 connected to the vacuum chamber through terminals 77 and 79 by means of high voltage coupling 78;. The terminal 77 is connected to the electrode 43 as well as to the shield or discharge electrode 20. The terminal 79 which is the high voltage or potential source V of FIG. 1 is connected over a conductor 82 to the extraction electrode 22 which is in turn connected to the intermediate electrode 23 through a voltage dropping resistor 83. This voltage dropping resistor 83 serves to maintain the extraction electrode 22 voltage at a normal V when subjected to high voltage discharge. This maintenance of the extraction electrode 22 at a V voltage prevents discharge over from the extraction electrode 22 to the field emission tip 21 and the resultant failure of the tip itself.

Assembly 102 of FIG. 2 constitutes an insertion device for aperture size control slide plate 110. Depending upon the beam size required for the specimen under investigation and for the desired resolution the mechanism of 102 comprising an adjustment screw with appropriate sealing members such as bellows and 0 rings permits movement of the aperture size plate across the axis of the housing electrodes until the desired size aperture is obtained. The lower chamber 103 is the specimen chamber itself. Within this housing, there is contained the specimen undergoing investigation as well as those detectors needed to obtain the necessary or desired information from the particle and specimens emissions. Chamber 103 is sealed to the main housing of the electron microscope through appropriate 0 ring sealingmembers. However, when a specimen change is required, in order to prevent contamination of the beam formation portions of the scanning electron microscope a mechanism 101 is employed. This mechanism 101 is similar to that of the aperture control mechanism 102 and comprises an adjustment screw arrangement with appropriate bellows and O ring sealing members to prevent leakage to the atmosphere. ln essence this mechanism 101 constitutes a valve unit which slides across the opening through which the beam passes in the lens 16 thereby preventing leakage to atmosphere of of the main portion of the chamber 41 when the specimen chamber 103 is vented.

lon pump 104 in combination with a sublimation getter pump 19 and an integral ion pumping action around and in the vicinity of the field emission tip 21 provides the necessary extremely low vacuum levels requisite to the proper performance of a field emission scanning electron microscope. Initial pumping to the level of approximately lO" is performed through the ion pump 104. When reaching this approximate level, the remainder of the pumping action is performed by getter material sublimed from a sublimation ring 19. Each is activated through the high voltage terminal 77. The

sublimed material condenses on the inner portion or surface of the discharge shield 20 and either buries contamination molecules within itself or reactively combines to remove them from the atmosphere surrounding the field emission tip 21. At the same time the electrons extracted from the field emission tip 21 cause ionization of gaseous molecules in its vicinity, which ions are then attracted to the discharge shield 20 and eventually burried within the sublimed material from the sublimator coil 19. This gettering pump action combined with the integral ion pumping contained within the field emission tip electrode configuration maintains the field emission tip 21 in a contamination free environment and holds that same environment at the minimum pressure contained within the entire system approximately 10' to 10" torr. This results in stable field emission tip 21 operation and greatly extended life and reliability. Chamber 100 surrounding the field emission tip assembly 50 is a chamber for holding cryogenic fluids resulting in a cryogenic pumping action again in the vicinity of the field emission tip 21. While this cryogenic tip action enhances the vacuum pumping action of the system, it is not necessary to proper performance of the equipment.

Considering the operational characteristics of a field emission scanning electron microscope employing the Applicants novel contribution to the art, reference will be had to FIG. 3 which shows a typical electrode field emission tip configuration and geometry. In analyzing this typical configuration it must be remembered that it is typical of that found in the systems of FIGS. 1 and 2 and therefore contains the same ancillary equipment and electrical connections to potential energy sources. In FIG. 3, there is shown a field emission tip axially aligned with apertures in the extraction electrode 22, intermediate electrode 23, and electrode 24, respectively. Further, there is shown the lens system 16 for stigmatic correction and deflection of the beam and the aperture control plate 110. In all scanning electron microscopes heretofore available an accelerating voltage between 5,000 and 100,000 volts was applied to the electrode 24, a potential between 500 and 3,500 volts or 2,000 volts nominal was applied to the intermediate electrode 23 and approximately the same 2,000 volt nominal potential was applied to the extraction electrode 22. These fields resulted in the formation of a beam of high intensity from the field emission tip 21 and produced a real focus of the beam at a distance S from the exit aperture of electrode 24. It is noted at this point that the electrode geometry configuration conforms to the optimum shapes as calculated by J. W. Butler and described in a paper by A. V. Crewe, D. N. Eggenberger, J. Wall, and L. M. Welter entitled Electron Gun Using a Field Emission Source, volume 39, No. 4, The Review of scientific Instruments, April, 1968, page 580. With electrode 24 and intermediate electrode 23 so configured; and with ratios of electrode 24 voltage V0 to intermediate electrode 23 voltage V1 between approximately and 30; and with axial electrode spacing of approximately 2 centimeters between 7 V0 is lowered toward a ratio of 10 with voltage Vl, the image distance S proceeds toward infinity and thus fails to produce a usable spot size for scanning electron microscopy operation. As this same ratio of V0 to V1 increases above approximately 30, the image distance S becomes negative and the focus is produced within the lens system comprising electrode 24 and intermediate electrode 23 and thus becomes unusable with respect to specimen investigation. In this range of voltage ratios between 10 and approximately 30, it is evident that the electron beam is one of high intensity due to the operating characteristics of the field emission tip 21 and of relatively high energy thereof presenting the difiiculties heretofore outlined in this specification.

The Applicants have discovered that as the ratio of V0 to V1 is continuously reduced that at the point V0 becomes of lower potential than V] thereby producing a negative field within the lens system formed by electrode 25 and intermediate electrode 23 with respect to an electron particle, a real focus of the beam reoccurs outside the lens field and distal of the field emission tip 21, obviously this focused beam is now usable for investigatory purposes with regard to the specimen. With regard to the particular lens configuration shown in FIG. 3, it is found that when the electrode 24 voltage V0 approaches approximately 300 volts with the intermediate electrode 23 voltage Vl at approximately 2,000 volts or in other words a ratio of approximately 0.15 a real image is formed approximately 5 centimeters from the exit aperture of the electrode 24, i.e., the image distance S equals 5 centimeters. The reason for the formation of this real focus at ratios of V0 to VI of less than unity is not fully understood. However without in any way limiting the scope of the applicants invention and without being bound in any way by the theories set forth herein, it is hypothesized that the negative field formed within the confines of electrode 24 and intermediate electrode 23 act upon the beam as a negative lens or mirror lens system and result in directing the beam electrons toward a focus in front of the electrode 24 when the beam comprises particles of sufficient energy to pass through the negative lens system and exit through the aperture of electrode 24. Macro-analysis of the effect of a mirror lens system conforming to that of the system presented in FIG. 3 shows the feasibility of the hypothesis set forth. However, it must be clearly understood that such analysis is overly simplistic and that the actual reasons and theories forming a basis for the Applicants discovery may be significantly more complex and of entirely different nature.

Operating a field emission scanning electron microscope in the novel mode, above set forth, allows the achieving of a small focussed spot of electrons accelerated to only a few hundred volts but with electron beam currents which are about the same as those obtained at higher accelerating voltages. When used in this new mode, the field emission gun still retains the advantages described and set forth in this application as well as previously referenced copending application, Ser. No. 46,425. When operating in this mode, the scanning electron microscope offers a number of advantages. The voltages needed to operate the gun being of much lower value obviate the need for complex design procedures to avoid voltage discharges which can damage parts or disrupt operation of the microscope. This necessarily results in a lower cost and a more diversified field of use for the equipment as well as enhanced reliability. Further the lower accelerating voltage V combined with the capability of producing a small spot size with high beam current allows the study of nonconducting specimens or objects without introducing the problem of charge up which is normally faced. It has been found that useful bright images can be obtained with V0 voltages as low as several hundred volts combined with tip voltages, i.e. V voltages of or times greater than this.

Yet another useful aspect of the applicants invention consists of its use in studying specimens which cannot be submitted to actual contact with the energetic beam. in FIG. 4 there is shown the specimen holding portion of a system arranged so as to avoid actual contact of the beam with the specimen 18. As shown, the beam focusses just above the surface of the specimen 18 and the deflected electrons are sensed by a detector 14 for display in the ancillary video receiver equipment 29. The specimen holder 15 and thus the specimen itself 18 is biased by a voltage source V30. This bias voltage being selected to a value which is equal in magnitude and opposite in polarity to the accelerating voltage V0 of the system such that the electrons of the beam 13 are decelerated to a stop just above the specimen or object surface. By carefully controlling the specimen 18 bias it is possible to detect the deflected scanning electrons as a function of the topography of the surface or electro static or magnetic properties of the surface. This constitutes a non-destructive test procedure or inspection device of the specimen 18 surface since the beam 13 electrons do not in any way interact at a high voltage with the specimen 18. Such mirror microscopy is made possible by the formation of the low energy beam with sufficient intensity to produce the desired resolution and contrast and finds particular value in instances where a hundred per cent inspection of parts is required but where no impingements of the beam may be allowed due to possible damage or interaction with the specimen. It also finds significant value when dealing with specimens that are of such a delicate nature that the energy of the beam itself may cause significant changes in the morphology of the substance.

Briefly in sum the Applicants have found that by applying an accelerating voltage V0 to the electrode 24 of a lower potential than the intermediate electrode 23 potential with respect to the charged particle comprising the beam to be focussed that there results an image of the beam formed at a point outside the focus field of the lens formed by electrode 24 and intermediate electrode 23 and distal of the field emission tip 21. The discovery of this mode for operation of a Field Emission Scanning Electron Microscope has resulted in a device of significantly broader application and increased reliability. It provides a microscopy instrument long sought by the industry capable of scanning the morphology and topography of a specimen without subjecting a specimen to high energy particle impingement. It is reiterated and emphasized that the applicants invention as set forth herein in the exemplary embodiments is intended to be illustrative of the apparatus and methods employed and is not intended to be delimiting of their scope. Thus, all those modifications and changes apparent to one skilled in the art are considered to be within the scope and ambit of the applicants invention.

What is claimed is:

l. A Field Emission Gun comprising:

a field emission tip for generating charged particles;

a final electrode axially aligned with said field emission tip;

an intermediate electrode interposed between said field emission tip and said final electrode forming a field lens with said final electrode for focusing said charged particles generated by said field emission tip;

and potential means for supplying voltages to said field emission tip, said final electrode and said intermediate electrode for providing extraction, acceleration, and focus fields, said final electrode potential being lower than said intermediate electrode potential with respect to said charged particles and the ratio of said final electrode potential to said intermediate electrode potential being selected such that said field lens provides a real focus of said charged particles at a position outside of said field lens and distal of said field emission tip without further lensing, thereby producing an intense beam of low energy particles.

2. The Field Emission Gun of claim 1 wherein the charged particles are electrons extracted from said field'emission tip by said extraction field.

3. A method for producing a low energy high intensity beam of charged particles from a field emission gun comprising the steps of:

applying extraction, accelerating and focusing potentials to an axially aligned field emission tip, a final electrode and an intermediate electrode interposed between field emission tip and said final electrode to form a beam of said charged particles;

and, selecting said final electrode potential sufficiently below said intermediate electrode potential with respect to said charged particles to form a lens field so as to produce a real focus of said charged particle beam outside said lens field and distal of said field emission tip without further focusing.

4. The method of claim 3 wherein said charged articles are electrons.

5. The method of claim 3 wherein said charged particles are electrons extracted from said field emission tip and wherein an extraction electrode is axially aligned with and in close proximity to said field emission tip and said extraction electrode voltage is approximately 2,000 volts, said intermediate electrode voltage is approximately 2,000 volts and said final electrode potential is approximately 300 volts.

6. The method of claim 3 wherein said charged particles are electrons extracted from said field emission tip and wherein an extraction electrode is axially aligned with said field emission tip and said extraction electrode and said intermediate electrode voltage is approximately at least 500 volts.

7. The method of claim 6 wherein the said final electrode, said extraction electrode and said intermediate electrode have apertures axially aligned with said field emission tip, said final electrode and said intermediate electrode form a lens field having an axial length of approximately 2 centimeters,

said extraction electrode is axially removed from said field emission tip by approximately I centimeter,

said final electrode potential is approximately volts, and said real focus occurs at a positionapproximately 5 centimeters from said final electrode aperature.

8. A field emission gun comprising:

a field emission tip for generatingelectrons;

a final electrode aligned with said field emission tip;

an intermediate electrode aligned with said field emission tip and interposed between said field emission tip and said final electrode forming a lens with said final electrode for focusing said electrons generated by said field emission tip;

an extraction electrode aligned with and in close proximity to said field emission tip providing an extraction field for drawing electrons from said field emission tip; and

potential means for supplying potentials to said field emission tip, said final electrode, said intermediate electrode and said extraction electrode for generating said extraction field and acceleration and focus fields, said final electrode potential being lower than said intermediate electrode potential and the ratio of said electrode potential to said intermediate electrode potential being selected such that said lens forms a real focus of said electrons without further lensing at a position outside of said lens and distal of said field emission tip thereby producing an intense beam of low energy electrons.

9. The Field Emission Gun of claim 8 wherein said final electrode said extraction electrode and said intermediate electrode have apertures axially aligned with said field emission tip through which said intense beam of low energy particles passes.

10. The field emission gun of claim 9 wherein said ratio is approximately 0.l5.

11. The field emission gun of claim 10 wherein the extraction field intensity is at least approximately 5 times 10 volts per centimeter.

12. The Field Emission Gun of claim 9 wherein said intermediate electrode voltage is at least approximately 500 volts, said field lens formed by said intermediate electrode and said final electrode has an axial length of approximately 2 centimeters, and said field emission tip is axially removed from said intermediate electrode a distance of approximately at least 1 centimeter.

13. The Field Emission Gun of claim 9 including integral pumping means operatively associated with said field emission tip for providing a contamination free environment for said field emission tip.

14. The Field Emission Gun of claim 13 including a sublimation pump for reactively combining with contaminant molecules in the environment of said field emission tip, and a cryogenic chamber for cryopumping contaminants from said environment of said field emission tip.

15. The field emission gun of claim 13 wherein said integral pumping means comprises electrons generated by said field emission tip and emitted from said extraction electrode for ionizing molecules in the area of said field emission tip.

16. The Field Emission Gun of claim 9 including biasing means for establishing the potential of a specimen located at said position for preventing impingement of said intense beam on said specimen. 

1. A Field Emission Gun comprising: a field emission tip for generating charged particles; a final electrode axially aligned with said field emission tip; an intermediate electrode interposed between said field emission tip and said final electrode forming a field lens with said final electrode for focusing said charged particles generated by said field emission tip; and potential means for supplying voltages to said field emission tip, said final electrode and said intermediate electrode for providing extraction, acceleration, and focus fields, said final electrode potential being lower than said intermediate electrode potential with respect to said charged particles and the ratio of said final electrode potential to said intermediate electrode potential being selected such that said field lens provides a real focus of said charged particles at a position outside of said field lens and distal of said field emission tip without further lensing, thereby producing an intense beam of low energy particles.
 2. The Field Emission Gun of claim 1 wherein the charged particles are electrons extracted from said field emission tip by said extraction field.
 3. A method for producing a low energy high intensity beam of charged particles from a field emission gun comprising the steps of: applying extraction, accelerating and focusing potentials to an axially aligned field emission tip, a final electrode and an intermediate electrode interposed between field emission tip and said final electrode to form a beam of said charged particles; and, selecting said final electrode potential sufficiently below said intermediate electrode potential with respect to said charged particles to form a lens field so as to produce a real focus of said charged particle beam outside said lens field and distal of said field emission tip without further focusing.
 4. The method of claim 3 wherein said charged articles are electrons.
 5. The method of claim 3 wherein said charged particles are electrons extracted from said field emission tip and wherein an extraction electrode is axially aligned with And in close proximity to said field emission tip and said extraction electrode voltage is approximately 2,000 volts, said intermediate electrode voltage is approximately 2,000 volts and said final electrode potential is approximately 300 volts.
 6. The method of claim 3 wherein said charged particles are electrons extracted from said field emission tip and wherein an extraction electrode is axially aligned with said field emission tip and said extraction electrode and said intermediate electrode voltage is approximately at least 500 volts.
 7. The method of claim 6 wherein the said final electrode, said extraction electrode and said intermediate electrode have apertures axially aligned with said field emission tip, said final electrode and said intermediate electrode form a lens field having an axial length of approximately 2 centimeters, said extraction electrode is axially removed from said field emission tip by approximately 1 centimeter, said final electrode potential is approximately 75 volts, and said real focus occurs at a position approximately 5 centimeters from said final electrode aperature.
 8. A field emission gun comprising: a field emission tip for generating electrons; a final electrode aligned with said field emission tip; an intermediate electrode aligned with said field emission tip and interposed between said field emission tip and said final electrode forming a lens with said final electrode for focusing said electrons generated by said field emission tip; an extraction electrode aligned with and in close proximity to said field emission tip providing an extraction field for drawing electrons from said field emission tip; and potential means for supplying potentials to said field emission tip, said final electrode, said intermediate electrode and said extraction electrode for generating said extraction field and acceleration and focus fields, said final electrode potential being lower than said intermediate electrode potential and the ratio of said electrode potential to said intermediate electrode potential being selected such that said lens forms a real focus of said electrons without further lensing at a position outside of said lens and distal of said field emission tip thereby producing an intense beam of low energy electrons.
 9. The Field Emission Gun of claim 8 wherein said final electrode said extraction electrode and said intermediate electrode have apertures axially aligned with said field emission tip through which said intense beam of low energy particles passes.
 10. The field emission gun of claim 9 wherein said ratio is approximately 0.15.
 11. The field emission gun of claim 10 wherein the extraction field intensity is at least approximately 5 times 10 7 volts per centimeter.
 12. The Field Emission Gun of claim 9 wherein said intermediate electrode voltage is at least approximately 500 volts, said field lens formed by said intermediate electrode and said final electrode has an axial length of approximately 2 centimeters, and said field emission tip is axially removed from said intermediate electrode a distance of approximately at least 1 centimeter.
 13. The Field Emission Gun of claim 9 including integral pumping means operatively associated with said field emission tip for providing a contamination free environment for said field emission tip.
 14. The Field Emission Gun of claim 13 including a sublimation pump for reactively combining with contaminant molecules in the environment of said field emission tip, and a cryogenic chamber for cryo-pumping contaminants from said environment of said field emission tip.
 15. The field emission gun of claim 13 wherein said integral pumping means comprises electrons generated by said field emission tip and emitted from said extraction electrode for ionizing molecules in the area of said field emission tip.
 16. The FiEld Emission Gun of claim 9 including biasing means for establishing the potential of a specimen located at said position for preventing impingement of said intense beam on said specimen. 